Spread spectrum transmission systems (transmission systems based on frequency spreading) can be used in the automotive sector, for example, particularly for communication between vehicle and a mobile transceiver, which is usually arranged in a vehicle key. A transmitter in the vehicle key can be used for the remote control of different functions, for example for locking and unlocking the doors of the vehicle (this application is also called “keyless entry”) and for remote-controlled starting of the motor, for activating the heating or air conditioning system, etc.
Remote control of some of these functions (e.g., activation of the independent vehicle heater or the air conditioning system) may employ a bidirectional data transmission by means of radio over relatively long distances (e.g., 2 to 4 km). Such applications are also called long range applications in the automotive sector.
In order to be able to attain the demanded ranges of 2 to 4 km (in the free field), relatively low data rates of approximately 2 kbit/s are usually used. The modulation method usually used in the automotive sector is frequency shift keying (FSK), or even Gaussian frequency shift keying (GFSK). Other modulation methods, for example amplitude shift keying (ASK), are also possible. Without further measures, conventional modulation techniques (e.g., frequency shift keying, FSK for short) result in correspondingly low RF bandwidths for the radio transmission on the basis of these low data rates.
In some countries (e.g., in the member states of the European Union), such “narrowband transmissions” are permitted up to an output power of +16.15 dBm EIRP (EIRP=equivalent isotropic radiated power). In other countries (e.g., in the USA), consistent “distribution” of the transmission power over a larger spectral bandwidth is prescribed in order to be able to transmit the required transmission powers (homologation limit: 30 dBm on 50 ohms, in practical application approximately 20 dBm). This requires the frequency spreading methods DSSS or FHSS (frequency hopping spread spectrum) (cf. FCC 15.249).
In order to comply with the different legal constraints in different countries (e.g., frequency spreading in the USA, narrowband transmission in Europe), different transmission systems (transmitter and receiver) may be necessary. For the narrowband transmission, the modulation method usually used in the automotive sector is frequency shift keying (FSK), particularly even Gaussian frequency shift keying (GFSK) at a data rate of approximately 2 kbit/s. Amplitude shift keying is also possible.